The present invention relates generally to DC a solid state switch which can be operated as a high-side switch which applies voltage to a load, or as a low-side switch which provides a closure to ground for a load, with built-in current limiting and test features relating to the state of operation.
Solid state switches for connecting a direct-current supply to a load are well known, as shown for example by U.S. Pat. No. 3,148,357 to Thornton and No. 3,610,953 to Gordon Schade, Jr., U.S. Pat. No. 4,274,014 shows a solid state switch in the form of a complementary symmetry inverter which include current limiting. In the patent current limiting is obtained with a current mirror transistor amplifier connected in series with the inverter and switched by one of the devices of the inverter. The switched current source not only limits the peak value of the switching current pulse but also limits the current between the inverter output and an external device. U.S. Pat. No. 4,188,547 to Fox is directed to a control circuit for a power switch or high level solid state relay. The Fox relay may be used for controlling the energization of a load from a DC power supply. Schmidt, Jr. in U.S. Pat. No. 3,720,848 discloses an analog switch or transmission gate which may be used as a solid state relay. Jorgensen in U.S. Pat. No. 3,984,703 discloses a CMOS Schmidtt Trigger. U.S. Pat. No. 3,789,244 to Provanzano is directed to a simple and inexpensive solid state switch which presents the same impedance pattern whether the circuit power is on or off, whether the signal at the input is in the normal range or is a substantial overvoltage, and irrespective of the polarity of the input signal.
A particular digital computer contained large numbers of electromagnetic relays. A solid state switch was needed which was efficient, required minimum printed-circuit board area, was low cost and had built in test features compatible with the digital computer.
Previous solid state switches were designed as either high-side switches or low-side switches. These were two distinctly different circuits and could not be used interchaneably.
Previous solid state switches required additional circuitry to inhibit the built-in test during a specified transient in order to prevent a false fault indication.